The present invention relates to a method for the heat treatment of finely granular material, in particular for the manufacture of cement, having a preferably multi-stage cyclone preheater, a rotary tubular kiln, a cooler, as well as a kiln off-gas line which is formed between the rotary tubular kiln and the cyclone preheater and is flown through substantially in upward direction by the off-gases of the rotary tubular kiln and to which tertiary air for the burning of an additional fuel is fed from the cooler, arranged on the material side behind the rotary tubular kiln, in two individual streams which are introduced at different heights into the kiln off-gas line.
A method of the type described above for the heat treatment of finely granular cement, for instance cement, lime, gypsum, dolomite, magnetite and other minerals which are to be thermally treated using different equipment is known. While in the one case the material to be treated flows in the same direction downward over the entire height through the precalcination zone formed by the kiln off-gas line as the off-gases coming from the rotary tubular kiln so that a very long heat treatment zone results, in another installation the material to be treated is fed to the rotary tubular kiln in countercurrent via the kiln off-gas line which forms a short precalcination zone. In this case, the kiln off-gas line is developed as a shaft of relatively large diameter from the upper part of which only the kiln off-gases emerge for further conveyance to the cyclone preheater.
For the carrying out of the precalcination there are used, in addition to the kiln off-gases of the rotary tubular kiln, also hot gases which come from the combustion by means of tertiary air of an additional fuel and which are withdrawn from the cooler arranged behind, with respect to the material, the rotary tubular kiln. In this connection it is known to feed the tertiary air either separately from the additional fuel to the lower end of the kiln off-gas line which forms the precalcination zone or to effect the combustion of the additional fuel by means of the tertiary air directly in front of the common entrance thereof into the kiln off-gas line, in which case there is the possibility of connecting a plurality of tertiary air lines at different heights to the kiln off-gas line and of feeding the corresponding proportion of additional fuel to each tertiary air line.
The known method with combustion of the additional fuel and precalcination taking place in the same direction of flow has various disadvantages. Since the calcination time is much shorter than the burnout time of the fuel and the oxygen partial pressure is drastically reduced by the mixing of tertiary air and kiln off-gas so that an additional increase in the burnout time is produced, considerable discrepancies arise upon unidirectional flow between combustion process and precalcination. These discrepancies are increased by the fact that, as a result of the simultaneous combustion and precalcination, CO.sub.2 is liberated from the thermal decomposition and further dilutes the concentration of the oxygen, thus further lengthening the burnout time.
Another disadvantage of the unidirectional flow process is that the alkalis and halogens liberated are transported together with the off-gases, into the cyclone preheater where they can lead to a clogging of the cyclone as a result of condensation and mineralization. This can be avoided, to be sure, by a suitably large partial gas discharge which extends parallel to the preheater but this means high heat losses.
The disadvantages described above are avoided in the known counter-flow method. In particular, in this known method the alkalis and halogens are retained in the kiln circuit, where they condense on the precalcined material fed to the rotary tubular kiln. As a result, a bypass system for the kiln off-gases is necessary only when there are very high contents of injurious substance.
However, both the unidirectional flow method and the counter-flow method have the disadvantage that the desired complete combustion of the additional fuel by means of the tertiary air fed from the cooler in the kiln off-gas line extending between rotary tubular kiln and cyclone preheater is prevented by the calcination which takes place at the same time and at the same place since this calcination process excludes the occurrence of temperatures of more than 820.degree. to 840.degree. C. If in the known methods the calcining was separated from the combustion of the additional fuel, high temperatures of more than 1600.degree. C. would be produced. At these temperatures, NO is formed in the off-gases and, upon the subsequent cooling of the gases, reacts with the excess of oxygen which is present for the complete combustion of the additional fuel, thus giving rise to injurious nitrogen oxides. These environment-contaminating nitrogen oxides (NO.sub.x) could be removed only by an expensive off-gas purification.